PURPOSE: To obtain a detector accurately detecting only damage of a cutting tool, by utilizing such relation that a generated AE signal from damage of the cutting rool contains a higher frequency component than a generated AE signal from destruction of a work under cutting.

CONSTITUTION: A detected signal in an AE sensor is input to a high pass filter 3A to detect only a high frequency component easily generatd at damage of a cutting tool. The signal is input to a squared circuit 6 and squared so that a less amount of damage may be detected. In this way, a level difference between the high frequency component and AE signal from destruction of a work is increased, and only damage of a level not less than a certain amount in the cutting tool can be accurately detected.